Semiconductor device and method of fabricating the same and method of forming nitride based semiconductor layer

ABSTRACT

A GaN layer is grown on a sapphire substrate, an SiO 2  film is formed on the GaN layer, and a GaN semiconductor layer including an MQW active layer is then grown on the GaN layer and the SiO 2  film using epitaxial lateral overgrowth. The GaN based semiconductor layer is removed by etching except in a region on the SiO 2  film, and a p electrode is then formed on the top surface of the GaN based semiconductor layer on the SiO 2  film, to join the p electrode on the GaN based semiconductor layer to an ohmic electrode on a GaAs substrate. An n electrode is formed on the top surface of the GaN based semiconductor layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor device having acompound semiconductor layer composed of BN (boron nitride), GaN(gallium nitride), AlN (aluminum nitride) or InN (indium nitride) or agroup III-V nitride compound semiconductor (hereinafter referred to as anitride based semiconductor) which is their mixed crystal and a methodof fabricating the same, and a method of forming a nitride basedsemiconductor layer.

[0003] 2. Description of the Background Art

[0004] In recent years, a GaN based light emitting semiconductor device(a light emitting semiconductor device based on GaN) has been put topractical use as a light emitting semiconductor device such as a lightemitting diode which emits light in blue or violet or a semiconductorlaser device. In fabricating the GaN based light emitting semiconductordevice, there exists no substrate composed of GaN. Therefore, each layeris epitaxially grown on an insulating substrate composed of sapphire(Al₂O₃) or the like.

[0005]FIG. 18 is a cross-sectional view showing the structure of aconventional GaN based light emitting diode. A light emitting diodeshown in FIG. 18 is disclosed in Nikkei Micro Device, February, 1994,pp. 92 to 93.

[0006] In FIG. 18, a GaN buffer layer 62, an n-GaN layer 63, an n-AlGaNcladding layer 64, an InGaN active layer 65, a p-AlGaN cladding layer66, and a p-GaN layer 67 are formed in this order on a sapphiresubstrate 61. A partial region from the p-GaN layer 67 to the n-GaNlayer 63 is removed by etching. A p electrode 68 is formed on the topsurface of the p-GaN layer 67, and an n electrode 69 is formed on theexposed top surface of the n-GaN layer 63. Such a structure of the lightemitting diode is referred to as a lateral structure.

[0007] The light emitting diode shown in FIG. 18 has a pn junctionhaving a double hetero structure in which the InGaN active layer 65 isinterposed between the n-AlGaN cladding layer 64 and the p-AlGaNcladding layer 66, and can emit light in blue.

[0008] In a conventional GaN based light emitting semiconductor deviceas shown in FIG. 18, however, dislocations of around 10 ⁹/cm² generallyexist in a GaN based semiconductor crystal which is grown on a sapphiresubstrate depending on the difference in lattice constant between GaNand the sapphire substrate. Such dislocations are propagated from thesurface of the sapphire substrate to a GaN based semiconductor layer. Inthe light emitting semiconductor device composed of the GaN basedsemiconductor layer on the sapphire substrate, device characteristicsand reliability are degraded due to the dislocations.

[0009] As a method of solving the problem of the degradation of thedevice characteristics and the reliability due to the dislocations,epitaxial lateral overgrowth has been proposed. The epitaxial lateralovergrowth is reported in “Proceedings of The Second InternationalConference on Nitride Semiconductors”, Oct. 27-31, 1997, Tokushima,Japan, pp.444-446. FIG. 19 is a schematic sectional view showing thesteps for explaining the conventional epitaxial lateral overgrowth.

[0010] As shown in FIG. 19 (a), an AlGaN buffer layer 82 is grown on asapphire substrate 81, and a GaN layer 83 is formed on the AlGaN bufferlayer 82. Dislocations 91 extending in the vertical direction exist inthe GaN layer 83. Striped SiO₂ films 90 are formed on the GaN layer 83.

[0011] As shown in FIG. 19 (b), a GaN layer 84 is regrown on the GaNlayer 83 exposed between the striped SiO₂ films 90. Also, in this case,the dislocations 91 extend in the vertical direction in the regrown GaNlayer 84.

[0012] As shown in FIG. 19 (c), when the GaN layer 84 is further grown,the GaN layer 84 is also grown in the lateral direction. Accordingly,the GaN layer 84 is also formed on the SiO₂ films 90. No dislocationsexist in the GaN layer 84 on the SiO₂ films 90.

[0013] As shown in FIG. 19 (d), when the GaN layer 84 is further grown,the GaN film 84 is formed on the SiO₂ films 90 and on the GaN layer 83between the SiO₂ films 90.

[0014] When the epitaxial lateral overgrowth is used, a GaN crystal ofhigh quality having no dislocations can be formed on the SiO₂ films 90.

[0015] In a region where the SiO₂ films 90 do not exist, however, thedislocations 91 from the underlying GaN layer 83 extend to the surfaceof the regrown GaN layer 84. Accordingly, the dislocations still existon the surface of the GaN layer 84. When the light emittingsemiconductor device is fabricated, therefore, a light emitting regionmust be limited to a region on the SiO₂ films. Therefore, it isimpossible to increase the size of the light emitting region.

[0016] When the area of the SiO₂ film is increased in order to increasethe area of the GaN layer of high quality, the surface of the GaN layerwhich is grown in the lateral direction cannot be flattened.

[0017] In the conventional GaN based light emitting diode shown in FIG.18, the sapphire substrate 61 is an insulating substrate. Therefore, then electrode 69 cannot be provided on the reverse surface of the sapphiresubstrate 61, and must be provided on the exposed surface of the n-GaNlayer 63. Therefore, a current path between the p electrode 68 and the nelectrode 69 is longer, so that an operation voltage is higher, ascompared with those in a case where the n electrode is provided on thereverse surface of a conductive substrate.

[0018] Furthermore, when a GaN based semiconductor laser device isfabricated, it is difficult to form cavity facets by a cleavage methodas in a semiconductor laser device for emitting red light or infraredlight using a GaAs substrate.

[0019]FIG. 20 is a diagram showing the relationship between the crystalorientations of a sapphire substrate and a GaN based semiconductorlayer. In FIG. 20, an arrow by a solid line indicates the crystalorientation of the sapphire substrate, and an arrow by a broken lineindicates the crystal orientation of the GaN based semiconductor layer.

[0020] As shown in FIG. 20, the a-axis and the b-axis of the GaN basedsemiconductor layer formed on the sapphire substrate are shifted 30°away from the a-axis and the b-axis of the sapphire substrate.

[0021]FIG. 21 is a schematic perspective view of a semiconductor laserdevice composed of a GaN based semiconductor layer formed on a sapphiresubstrate.

[0022] In FIG. 21, a GaN based semiconductor layer 70 is formed on a(0001) plane of a sapphire substrate 61. A striped current injectionregion 71 is parallel to a <1120> direction of the GaN basedsemiconductor layer 70. In this case, a {1100} plane of the GaN basedsemiconductor layer 70 is inclined at 30° to a {1100} plane of thesapphire substrate 61. Both the sapphire substrate 61 and the GaN basedsemiconductor layer 70 are easily cleaved along the {1100} plane.

[0023] The respective cleavage directions of the sapphire substrate 61and the GaN based semiconductor layer 70 thus deviate. When a GaN basedsemiconductor laser device is fabricated, therefore, it is difficult toform cavity facets by a cleavage method, as in the semiconductor laserdevice for emitting red light or infrared light which is formed on theGaAs substrate. In this case, the necessity of forming the cavity facetsby etching is brought about. When the cavity facets are formed byetching, however, it is impossible to reduce an operation current of thesemiconductor laser device because it is difficult to form facetsperpendicular to the substrate.

[0024] On the other hand, various reports and proposals are made withrespect to methods of controlling a transverse mode of the GaN basedsemiconductor laser device. Almost all of the methods of controlling thetransverse mode include two types, i.e., a ridge waveguided structureand a self-aligned structure which are employed by the conventionalsemiconductor laser device for emitting red light or infrared light.

[0025] Since the GaN based semiconductor layer is chemically stable,however, it cannot be patterned by wet etching, unlike an AlGaAs basedsemiconductor layer used for the conventional semiconductor laser deviceemitting red light or infrared light, and must be patterned by dryetching such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion BeamEtching).

[0026] In the GaN based semiconductor laser device, therefore, thepatterning for fabricating the ridge waveguided structure or theself-aligned structure cannot be performed easily and with goodreproducibility. Moreover, device characteristics greatly vary dependingon the precision of the dry etching.

SUMMARY OF THE INVENTION

[0027] An object of the present invention is to provide a nitride basedsemiconductor device (a semiconductor device based on a nitride) capableof performing a low-voltage operation.

[0028] Another object of the present invention is to provide a nitridebased light emitting semiconductor device capable of performing alow-voltage operation and forming a facet by cleavage.

[0029] Still another object of the present invention is to provide amethod of fabricating a nitride based semiconductor device in which thenumber of dislocations is reduced, a low-voltage operation can beperformed, and a facet can be formed by cleavage.

[0030] Still another object of the present invention is to provide amethod of forming a nitride based semiconductor layer (a semiconductorlayer based on a nitride) of high quality having no dislocations in awide region on a substrate.

[0031] Still another object of the present invention is to provide asemiconductor device composed of a nitride based semiconductor layer ofhigh quality having no dislocations in a wide region on a substrate.

[0032] Still another object of the present invention is to provide alight emitting semiconductor device having an active layer of highquality having no dislocations in a wide region on a substrate.

[0033] Still another object of the present invention is to provide anitride based semiconductor device capable of fabricating an activeregion of the device of high quality without using etching.

[0034] Still another object of the present invention is to provide arefractive index guided nitride based light emitting semiconductordevice capable of forming an optical waveguide path of high qualitywithout using etching.

[0035] Still another object of the present invention is to provide amethod of fabricating a nitride based semiconductor device capable offabricating an active region of the device of high quality without usingetching.

[0036] A semiconductor device according to one aspect of the presentinvention comprises a gallium arsenide substrate, a first electrodelayer formed on the gallium arsenide substrate, a nitride basedsemiconductor layer formed on the first electrode layer and containingat least one of boron, gallium, aluminum and indium, and a secondelectrode layer formed on the nitride based semiconductor layer.

[0037] In the semiconductor device, the first electrode layer and thesecond electrode layer are respectively provided on the bottom surfaceand the top surface of the nitride based semiconductor layer on thegallium arsenide substrate. Accordingly, a current path between thefirst electrode layer and the second electrode layer is shortened.Consequently, it is possible to reduce an operation voltage of thenitride based semiconductor device.

[0038] The nitride based semiconductor layer may include an activelayer. In this case, the first electrode layer and the second electrodelayer are respectively provided on the bottom surface and the topsurface of the nitride based semiconductor layer on the gallium arsenidesubstrate. Accordingly, the current path between the first electrodelayer and the second electrode layer is shortened. Consequently, it ispossible to reduce the operation voltage of the nitride based lightemitting semiconductor device.

[0039] Particularly, it is preferable that the nitride basedsemiconductor layer has a striped current injection region for injectinga current into the active layer, the striped current injection region isformed along a <1100> direction of the nitride based semiconductorlayer, the nitride based semiconductor layer is arranged on the galliumarsenide substrate such that the <1100> direction of the nitride basedsemiconductor layer coincides with a <110> direction or a <110>direction of the gallium arsenide substrate, and a pair of cavity facetsis formed of a {110} plane or a {110} plane of the gallium arsenidesubstrate and a {1100} plane of the nitride based semiconductor layer.

[0040] In this case, the cleavage direction of the nitride basedsemiconductor layer and the cleavage direction of the gallium arsenidesubstrate coincide with each other. Accordingly, the cavity facets canbe formed by the cleavage. Therefore, a semiconductor laser device isrealized as a light emitting semiconductor device. Further, theuniformity of device characteristics is improved, and thereproducibility of the device characteristics is increased.

[0041] A method of fabricating a semiconductor device according toanother aspect of the present invention comprises the steps of forming afirst nitride based semiconductor layer containing at least one ofboron, gallium, aluminum and indium on an insulating substrate, formingan insulating film in a predetermined region on the first nitride basedsemiconductor layer, forming a second nitride based semiconductor layercontaining at least one of boron, gallium, aluminum and indium usingepitaxial lateral overgrowth on the first nitride based semiconductorlayer and the insulating film, removing the second nitride basedsemiconductor layer except in a region on the insulating film, joiningthe top surface of the second nitride based semiconductor layer on theinsulating film to one surface of a gallium arsenide substrate through afirst electrode layer, removing the insulating film, to remove theinsulating substrate and the first nitride based semiconductor layerfrom the second nitride based semiconductor layer, and forming a secondelectrode layer on the second nitride based semiconductor layer.

[0042] In the method of fabricating the semiconductor device, the secondnitride based semiconductor layer is formed using the epitaxial lateralovergrowth on the insulating film on the first nitride basedsemiconductor layer. Accordingly, no dislocations are propagated to thesecond nitride based semiconductor layer on the insulating film from thefirst nitride based semiconductor layer. Consequently, a second nitridebased semiconductor layer of high quality having few dislocations isobtained.

[0043] The first electrode layer and the second electrode layer arerespectively provided on the bottom surface and the top surface of thesecond nitride based semiconductor layer on the gallium arsenidesubstrate. Accordingly, a current path between the first electrode layerand the second electrode layer is shortened. Consequently, an operationvoltage can be reduced.

[0044] Furthermore, when the top surface of the second nitride basedsemiconductor layer is joined to one surface of the gallium arsenidesubstrate through the first electrode layer, the crystal orientations ofthe second nitride based semiconductor layer and the gallium arsenidesubstrate can be matched with each other. Accordingly, a facet of thesemiconductor device can be formed by cleavage. Consequently, theuniformity of device characteristics is improved, and thereproducibility of the device characteristics is increased.

[0045] Particularly, the step of forming the second nitride basedsemiconductor layer may comprise the step of forming an active layer.Consequently, a light emitting semiconductor device is fabricated as thesemiconductor device.

[0046] Furthermore, the step of forming the second nitride basedsemiconductor layer may further comprise the step of forming a stripedcurrent injection region for injecting a current into the active layeralong a <1100> direction of the second nitride based semiconductorlayer, and the step of joining the top surface of the second nitridebased semiconductor layer to one surface of the gallium arsenidesubstrate through the first electrode layer may comprise the step ofmatching the <1100> direction of the second nitride based semiconductorlayer with a <110> direction or a <110> direction of the galliumarsenide substrate. The fabricating method may further comprise the stepof forming a pair of cavity facets by cleavage along a {110} plane or a{110} plane of the gallium arsenide substrate and a {1100} plane of thesecond nitride based semiconductor layer.

[0047] In this case, the cleavage direction of the nitride basedsemiconductor layer and the cleavage direction of the gallium arsenidesubstrate coincide with each other. Accordingly, the cavity facets canbe formed by the cleavage. Consequently, a semiconductor laser device isrealized as the light emitting semiconductor device. Further, the devicecharacteristics hardly vary, and the reproducibility of the devicecharacteristics is increased.

[0048] A method of forming a nitride based semiconductor layer accordingto still another aspect of the present invention comprises the steps offorming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate,forming an irregular pattern including a recess having a bottom surfaceformed of an insulator and a projection having a top surface formed ofan insulator in the surface of the first nitride based semiconductorlayer, and forming a second nitride based semiconductor layer containingat least one of boron, gallium, aluminum and indium on the insulators bygrowth from the first nitride based semiconductor layer using epitaxiallateral overgrowth.

[0049] In the method of forming the nitride based semiconductor layer,the insulators are formed on the bottom surface of the recess in theirregular pattern formed on the surface of the first nitride basedsemiconductor layer and the top surface of the projection in theirregular pattern. Accordingly, the first nitride based semiconductorlayer is exposed to only side surfaces of the irregular pattern.Therefore, the second nitride based semiconductor layer is formed on theinsulators by the growth in the lateral direction of the nitride basedsemiconductor layer. Consequently, no dislocations are propagated to thesecond nitride based semiconductor layer from the first nitride basedsemiconductor layer. As a result, a nitride based semiconductor layer ofhigh quality having no dislocations is formed in a wide region on thesubstrate. The step of forming the irregular pattern may comprise thesteps of forming the irregular pattern such that the first nitride basedsemiconductor layer is exposed to the bottom surface of the recess, andforming an insulating film as the insulator on the bottom surface of therecess of the irregular pattern and forming an insulating film as theinsulator on the top surface of the projection of the irregular pattern.

[0050] Alternatively, the step of forming the irregular pattern maycomprise the step of forming an insulating film as the insulator in aregion on the first nitride based semiconductor layer where theprojection of the irregular pattern is to be formed and removing thefirst nitride based semiconductor such that the insulating substrate isexposed as the insulator except in a region on the insulating film.

[0051] Particularly, it is preferable that the irregular pattern has astriped recess and a striped projection which extend along a <1120>direction of the first nitride based semiconductor layer. Consequently,the nitride based semiconductor layer is easily grown in the lateraldirection.

[0052] It is preferable that a cross-sectional shape of the projectionof the irregular pattern is a rectangular shape or a reversed mesa shapehaving vertical side surfaces. When the insulating films are depositedon the irregular pattern, therefore, the insulating films can be formedonly on the bottom surface of the recess in the irregular pattern andthe top surface of the projection in the irregular pattern. Therefore,it is possible to omit the step of removing the insulating films on theside surfaces of the irregular pattern.

[0053] A semiconductor device according to still another aspect of thepresent invention comprises an insulating substrate, a first nitridebased semiconductor layer formed on the insulating substrate andcontaining at least one of boron, gallium, aluminum and indium, anirregular pattern being formed in the surface of the first nitride basedsemiconductor layer, insulating films respectively formed on the bottomsurface of a recess and the top surface of a projection of the irregularpattern of the first nitride based semiconductor layer, and a secondnitride based semiconductor layer formed on the insulating films andcontaining at least one of boron, gallium, aluminum and indium.

[0054] In the semiconductor device, the insulating films arerespectively formed on the bottom surface of the recess in the irregularpattern on the surface of the first nitride based semiconductor layerand the top surface of the projection in the irregular pattern, and thesecond nitride based semiconductor layer is formed on the insulatingfilms. Accordingly, no dislocations are propagated to the second nitridebased semiconductor layer from the first nitride based semiconductorlayer. Consequently, a nitride based semiconductor device of highquality having no dislocations is realized in a wide region on thesubstrate.

[0055] The second nitride based semiconductor layer may include anactive layer. In this case, the insulating films are respectively formedon the bottom surface of the recess in the irregular pattern on thesurface of the first nitride based semiconductor layer and the topsurface of the projection in the irregular pattern, and the secondnitride based semiconductor layer is formed on the insulating films.Accordingly, no dislocations are propagated to the second nitride basedsemiconductor layer from the first nitride based semiconductor layer.Consequently, a nitride based semiconductor device including an activelayer of high quality having no dislocations is realized in the wideregion on the substrate.

[0056] A semiconductor device according to still another aspect of thepresent invention comprises an insulating substrate, a first nitridebased semiconductor layer formed on the insulating substrate andcontaining at least one of boron, gallium, aluminum and indium, aplurality of striped insulating films formed a predetermined distanceaway from each other on the first nitride based semiconductor layer, anda second nitride based semiconductor layer formed on the first nitridebased semiconductor layer and the plurality of striped insulating filmsand containing at least one of boron, gallium, aluminum and indium. Thesecond nitride based semiconductor layer includes an active region ofthe device above the plurality of striped insulating films. The activeregion is an actually operating region other than an electrode formingregion of the semiconductor device.

[0057] In the semiconductor device, the second nitride basedsemiconductor layer is formed on the first nitride based semiconductorlayer through the plurality of striped insulating films. Accordingly,few dislocations exist in the second nitride based semiconductor layeron a region where the plurality of striped insulating films exist.Consequently, the active region of the device above the plurality ofstriped insulating films is increased in quality.

[0058] On the region where the plurality of striped insulating filmsexist, the growth rate of the second nitride based semiconductor layeris lower, as compared with that on the region where the plurality ofstriped insulating films do not exist. Consequently, the second nitridebased semiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate the active region of the device without using etching.

[0059] The active region may include a light emitting portion. In thiscase, the second nitride based semiconductor layer is formed on thefirst nitride based semiconductor layer through the plurality of stripedinsulating films. Accordingly, few dislocations exist in the secondnitride based semiconductor layer on the region where the plurality ofstriped insulating films exist. Consequently, the light emitting portionabove the plurality of striped insulating films is increased in quality.

[0060] On the region where the plurality of striped insulating filmsexist, the growth rate of the second nitride based semiconductor layeris lower, as compared with that on the region where the plurality ofstriped insulating films do not exist. Consequently, the second nitridebased semiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate a refractive index guided optical waveguide path withoutusing etching.

[0061] Particularly, the second nitride based semiconductor layer mayhave a striped current injection region for injecting a current into thelight emitting portion, and the second nitride based semiconductor layermay comprise a pair of cavity facets perpendicular to the stripedcurrent injection region. Consequently, the current is injected in astripe shape into the light emitting portion, thereby making lasingpossible.

[0062] It is preferable that the plurality of striped insulating filmsare formed along a <1120> direction of the first nitride basedsemiconductor layer. Consequently, the nitride based semiconductor layeris easily grown in the lateral direction, and the second nitride basedsemiconductor layer is made higher in quality.

[0063] A method of fabricating a semiconductor device according to stillanother aspect of the present invention comprises the steps of forming afirst nitride based semiconductor layer containing at least one ofboron, gallium, aluminum and indium on an insulating substrate, forminga plurality of striped insulating films a predetermined distance awayfrom each other on the first nitride based semiconductor layer, andforming a second nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium using epitaxial lateralovergrowth on the first nitride based semiconductor layer and theplurality of striped insulating films, to form an active region of thedevice above the plurality of striped insulating films. The activeregion is an actually operating region other than an electrode formingregion of the semiconductor device.

[0064] In the method of fabricating the semiconductor device, the secondnitride based semiconductor layer is formed using epitaxial lateralovergrowth through the plurality of striped insulating films on thefirst nitride based semiconductor layer. Accordingly, few dislocationsexist in the second nitride based semiconductor layer on the regionwhere the plurality of striped insulating films exist. Consequently, theactive region of the device above the plurality of striped insulatingfilms is increased in quality.

[0065] On the region where the plurality of striped insulating filmsexist, the growth rate of the second nitride based semiconductor layeris lower, as compared with that on the region where the plurality ofstriped insulating films do not exist. Consequently, the second nitridebased semiconductor layer on the region where the plurality of stripedinsulating films exist is concavely formed. Consequently, it is possibleto fabricate an active layer of the device without using etching.

[0066] A method of forming a nitride based semiconductor layer accordingto still another aspect of the present invention comprises the steps offorming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate,forming an irregular pattern having exposed side surfaces in the firstnitride based semiconductor layer, and forming a second nitride basedsemiconductor layer containing at least one of boron, gallium, aluminumand indium on the irregular pattern by growth from the exposed sidesurfaces of the irregular pattern on the first nitride basedsemiconductor layer using epitaxial lateral overgrowth.

[0067] In the method of forming the nitride based semiconductor layer,the side surfaces of the irregular pattern formed in the first nitridebased semiconductor layer are exposed. Therefore, the second nitridebased semiconductor layer is formed on the irregular pattern by growthin the lateral direction of the nitride based semiconductor layer fromthe exposed side surfaces. Consequently, no dislocations are propagatedto the second nitride based semiconductor layer from the first nitridebased semiconductor layer. As a result, a nitride based semiconductorlayer of high quality having no dislocations is formed in a wide regionon the substrate.

[0068] The step of forming the irregular pattern may comprise the stepsof forming the irregular pattern such that the first nitride basedsemiconductor layer is exposed to the bottom surface of a recess, andforming insulating films on the bottom surface of the recess of theirregular pattern and the top surface of the projection of the irregularpattern.

[0069] Alternatively, the step of forming the irregular pattern maycomprise the step of forming an insulating film in a region on the firstnitride based semiconductor layer where the projection in the irregularpattern is to be formed, and removing the first nitride basedsemiconductor such that the insulating substrate is exposed except in aregion on the insulating film.

[0070] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjuncitonwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071]FIG. 1 is a schematic sectional view showing the steps of a methodof fabricating a semiconductor laser device in a first embodiment of thepresent invention;

[0072]FIG. 2 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the firstembodiment of the present invention;

[0073]FIG. 3 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the firstembodiment of the present invention;

[0074]FIG. 4 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the firstembodiment of the present invention;

[0075]FIG. 5 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the firstembodiment of the present invention;

[0076]FIG. 6 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the firstembodiment of the present invention;

[0077]FIG. 7 is a diagram showing an energy band structure of an MQWactive layer in the semiconductor laser device shown in FIGS. 1 to 6;

[0078]FIG. 8 is a perspective view showing the relationship between thecrystal orientations of a sapphire substrate and a GaN basedsemiconductor layer in the semiconductor laser device shown in FIGS. 1to 6;

[0079]FIG. 9 is a schematic sectional view showing the steps of a methodof forming a GaN based semiconductor layer in a second embodiment of thepresent invention;

[0080]FIG. 10 is a schematic sectional view showing the steps of themethod of forming the GaN based semiconductor layer in the secondembodiment of the present invention;

[0081]FIG. 11 is a schematic sectional view showing an irregular patternin a reversed mesa shape formed on the surface of a GaN layer;

[0082]FIG. 12 is a schematic sectional view showing a method of formingthe irregular pattern in a reversed mesa shape on the surface of the GaNlayer;

[0083]FIG. 13 is a schematic sectional view showing an example of asemiconductor laser device fabricated on the GaN layer formed by themethod shown in FIGS. 9 and 10;

[0084]FIG. 14 is a schematic sectional view showing the steps in anotherexample of a method of forming a GaN based semiconductor layer;

[0085]FIG. 15 is a schematic sectional view showing the steps in anotherexample of the method of forming the GaN based semiconductor layer;

[0086]FIG. 16 is a schematic sectional view showing the steps of amethod of fabricating a semiconductor laser device in a third embodimentof the present invention;

[0087]FIG. 17 is a schematic sectional view showing the steps of themethod of fabricating the semiconductor laser device in the thirdembodiment of the present invention;

[0088]FIG. 18 is a schematic sectional view of a conventional GaN basedlight emitting diode;

[0089]FIG. 19 is a schematic sectional view showing the steps of aconventional method of forming a GaN based semiconductor layer usingepitaxial lateral overgrowth;

[0090]FIG. 20 is a diagram showing the relationship between the crystalorientations of a sapphire substrate and a GaN based semiconductor layerformed thereon; and

[0091]FIG. 21 is a diagram showing the relationship between the crystalorientations of the sapphire substrate and the GaN based semiconductorlayer formed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0092] FIGS. 1 to 6 are schematic sectional views showing the steps of amethod of fabricating a semiconductor laser device in a first embodimentof the present invention.

[0093] As shown in FIG. 1, an AlGaN buffer layer 2 is first formed on asapphire substrate 1, and an undoped GaN layer 3 is grown on the AlGaNbuffer layer 2. An SiO₂ film 4 having a predetermined width is formed onthe GaN layer 3, and an n-GaN layer 5 is then grown on the GaN layer 3and the SiO₂ film 4 using epitaxial lateral overgrowth.

[0094] An n-In_(P)Ga_(1−P)N (P=0.1) crack preventing layer 6 having athickness of 0.1 μm, an n-Al_(Y)Ga_(1−Y)N (Y=0.07) cladding layer 7having a thickness of 1.0 μm, a multi quantum well active layer(hereinafter referred to as an MQW active layer) 8 described later, ap-Al_(Y)Ga_(1−Y)N (Y=0.07) cladding layer 9 having a thickness of 0.15μm, and an n-Al_(Z)Ga_(1−Z)N (Z=0.12) current blocking layer 10 having athickness of 0.20 μm are then formed in this order on the n-GaN layer 5.

[0095]FIG. 7 is a diagram showing an energy band structure of the MQWactive layer 8. As shown in FIG. 7, the MQW active layer 8 includes amulti quantum well structure obtained by alternately laminating sixIn_(X)Ga_(1−X)N (X=0.03) quantum barrier layers 81 having a thickness of60 Å and five In_(X)Ga_(1−X)N (X=0.10) quantum well layers 82 having athickness of 30 Å. Both surfaces of the multi quantum well structure areinterposed between GaN optical guide layers 83 having a thickness of 0.1μm.

[0096] A striped region having a width W0 at the center of then-Al_(Z)Ga_(1−Z)N current blocking layer 10 is then removed by etching.In this case, the striped region between the n-Al_(Z)Ga_(1−Z)N currentblocking layers 10 is a current injection region 19. The width W0 of thecurrent injection region 19 is 2 μm, for example. The current injectionregion 19 is formed in a <1120> direction of GaN.

[0097] Furthermore, a p-Al_(Y)Ga_(1−Y)N (Y=0.07) cladding layer 11having a thickness of 0.4 μm and a p-GaN contact layer 12 having athickness of 0.1 μm are formed in this order on the n-Al_(Z)Ga_(1−Z)Ncurrent blocking layer 10 and the p-Al_(Y)Ga_(1−Y)N cladding layer 9.

[0098] Si is used as an n-type dopant, and Mg is used as a p-typedopant. Examples of a method of growing each layer include MOCVD (MetalOrganic Chemical Vapor Deposition) or HVPE (Hydride Vapor PhaseEpitaxy).

[0099] In this case, dislocations extend in the vertical direction fromthe GaN layer 3 to the p-GaN contact layer 12 in a region where the SiO₂film 4 does not exist. No dislocations exist from the n-GaN layer 5 tothe p-GaN contact layer 12 on the SiO₂ film 4.

[0100] As shown in FIG. 2, the region, from the p-GaN contact layer 12to the n-GaN layer 5, where the SiO₂ film 4 does not exist are removedby dry etching such as RIE or RIBE. Consequently, a GaN basedsemiconductor layer 18 having no dislocations remains on the GaN layer3.

[0101] Furthermore, in order to activate the p-type dopant, a pelectrode 13 composed of Ni having a thickness of 5000 Å, Pt having athickness of 100 Å, and Au having a thickness of 1 μm is formed on thep-GaN contact layer 12, as shown in FIG. 3, after annealing at atemperature over 600° C., for example, at 800° C. for 30 minutes.

[0102] As shown in FIG. 4, an n-GaAs substrate 14 of 100 μm thicknesshaving ohmic electrodes 15 a and 15 b formed on its top and reversesurfaces of (001) planes is prepared. The top surface of the p electrode13 formed on the GaN based semiconductor layer 18 on the sapphiresubstrate 1 is joined by thermocompression (bonding) or fusion to theohmic electrode 15 a on the n-GaAs substrate 14.

[0103] When the thermocompression is used, it is desirable that thesurface of the p electrode 13 and the surface of the ohmic electrode 15a are covered with Au in a state immediately after evaporation. When thefusion is used, it is desirable that an Au—Sn film having a thickness ofapproximately 3 μm is formed on the n-GaAs substrate 14.

[0104] In joining the p electrode 13 and the ohmic electrode 15 a, therespective crystal orientations of the GaAs substrate 14 and the GaNbased semiconductor layer 18 are matched with each other such that then-GaAs substrate 14 and the GaN based semiconductor layer 18 have arelationship shown in FIG. 8.

[0105] In FIG. 8, the GaN based semiconductor layer 18 is formed on a(001) plane of the n-GaAs substrate 14. A striped current injectionregion 19 corresponds to a region between the n-Al_(Z)Ga_(1−Z)N currentblocking layers 10 shown in FIG. 4, and a light emitting portion 20 isformed in the current injection region 19. The current injection region19 is provided in a <1100> direction. In this case, the GaN basedsemiconductor layer 18 is joined to the n-GaAs substrate 14 such thatthe current injection region 19 in the GaN based semiconductor layer 18is parallel to a <110> direction or a <110> direction of the n-GaAssubstrate 14.

[0106] The sapphire substrate 1 and the n-GaAs substrate 14 shown inFIG. 4 are then dipped in a raw hydrofluoric acid, to remove the SiO₂film 4. The sapphire substrate 1 and the AlGaN buffer layer 2 and theGaN layer 3, having the dislocations, on the sapphire substrate 1 areremoved from the GaN based semiconductor layer 18 on the n-GaAssubstrate 14 by lift-off. Consequently, the GaN based semiconductorlayer 18 having no dislocations remains on the n-GaAs substrate 14, asshown in FIG. 5. In this case, it is desirable that a raw hydrofluoricacid containing a surface active agent is used such that side etching ofthe SiO₂ film 4 easily progresses.

[0107] Finally, as shown in FIG. 6a, an SiO₂ film 16 for preventingshort is formed on the top surface and side surfaces of the GaN basedsemiconductor layer 18 and the surface of the ohmic electrode 15 aexcept in a region at the center of the n-GaN layer 5, and an nelectrode 17 composed of Ti having a thickness of 100 Å and Al having athickness of 2000 Å is formed on the n-GaN layer 5 and the SiO₂ film 16.

[0108] Thereafter, a pair of cavity facets is formed by a cleavagemethod. In this case, a {1100} plane of the GaN based semiconductorlayer 18 and a {110} plane or a {110} plane of the n-GaAs substrate 14are cleavage planes, as shown in FIG. 8.

[0109] In the semiconductor laser device according to the presentembodiment, the p electrode 13 and the n electrode 17 are respectivelyformed on the reverse surface and the surface of the GaN basedsemiconductor layer 18. Accordingly, a current path between the pelectrode 13 and the n electrode 17 is shortened. Further, fewdislocations exist in the GaN based semiconductor layer 18.Consequently, a low-voltage operation and a low-current operation arepossible.

[0110] The cleavage direction of the GaN based semiconductor layer 18and the cleavage direction of the n-GaAs substrate 14 can be matchedwith each other. Accordingly, the cavity facets can be easily formed bythe cleavage method.

[0111] Although in the present embodiment, description was made of acase where the present invention is applied to the semiconductor laserdevice, the present invention is also applicable to light emittingsemiconductor devices such as a light emitting diode and the othersemiconductor devices.

[0112]FIGS. 9 and 10 are schematic sectional views showing the steps ofa method of forming a GaN based semiconductor layer in a secondembodiment of the present invention.

[0113] The top surface of a sapphire substrate 21 shown in FIG. 9 (a)has a (0001) plane (c face). As shown in FIG. 9 (b), an AlGaN bufferlayer 22 and an updoped GaN layer 23 are grown in this order on the(0001) plane of the sapphire substrate 21. Dislocations 37 extending inthe vertical direction exist in the GaN layer 23.

[0114] As shown in FIG. 9 (c), the GaN layer 23 is then etched by RIEusing striped masks 29 composed of Ni, to form a striped irregularpattern on the surface of the GaN layer 23. The respective widths D of arecess and a projection in the irregular pattern shall be 5 μm, forexample.

[0115] After the striped masks 29 are removed, an SiO₂ film 30 is thenformed on the GaN layer 23, as shown in FIG. 9 (d).

[0116] As shown in FIG. 10 (e), the SiO₂ film 30 formed on side surfacesof the irregular pattern on the GaN layer 23 is then removed by etching.

[0117] Thereafter, as shown in FIG. 10 (f), a GaN layer 24 is regrown.At this time, the underlying GaN layer 23 is exposed only to the sidesurfaces of the irregular pattern. When the regrowth of the GaN layer 24is started, therefore, the GaN layer 24 is not grown in the verticaldirection and is grown only in the lateral direction. The dislocations37 in the underlying GaN layer 23 are not propagated to the GaN layer 24which is grown in the lateral direction on the SiO₂ films 30.

[0118] As shown in FIG. 10 (g), as the GaN layer 24 is regrown, the SiO₂films 30 on the lower step of the irregular pattern are filled in withthe GaN layer 24. The GaN layer 24 is grown in the vertical direction.

[0119] Thereafter, as shown in FIG. 10 (h), the GaN layer 24 is grown inthe lateral direction as well as the vertical direction on the SiO₂films 30 on the upper step of the irregular pattern. Accordingly, thesurface of the GaN layer 24 is flattened. Consequently, the GaN layer 24of high quality having no dislocations is formed on the SiO₂ films 30 onthe irregular pattern.

[0120] In order to flatten the surface of the GaN layer 24 which isregrown, the GaN layer 24 must have a certain degree of thickness. Thethickness required to flatten the surface of the GaN layer 24 differsdepending on the growth conditions such as the width of the irregularpattern on the underlying GaN layer 23 and the substrate temperature atthe time of the growth of the GaN layer 24. For example, when therespective widths of the recess and the projection in the irregularpattern are approximately 5 μm, the thickness of the GaN layer 24 mustbe approximately 10 to 20 μm.

[0121] GaN is easily grown in a <1100> direction. In order that the GaNlayer 24 is easily grown in the lateral direction in the steps shown inFIGS. 10 (f), 10 (g), and 10 (h), therefore, it is desirable that thestriped masks 29 composed of Ni are formed in a <1120> directionperpendicular to the <1100> direction of the GaN layer 23 in the stepshown in FIG. 9 (c).

[0122] In order that the surface of the GaN layer 24 which is regrown inthe step shown in FIG. 10 (h) is easily flattened, the mask width of thestriped masks 29 composed of Ni and the window width of the stripedmasks 29 (the width of a region where no Ni exists) which are used inthe step shown in FIG. 9 (c) are preferably as small as not more than 10μm, and more preferably 1 to 5 μm.

[0123] Furthermore, it is preferable that a cross-sectional shape of theprojection in the irregular pattern on the underlying GaN layer 23 is arectangular shape or a reversed mesa shape (reversed trapezoidal shape)having vertical side surfaces rather than a mesa shape (trapezoidalshape).

[0124] As shown in FIG. 11, the cross sectional shape of the irregularpattern on the GaN layer 23 is a reversed mesa shape, and the SiO₂ films30 are formed on the GaN layer 23 by a deposition method, such aselectron beam evaporation, inferior in step coverage, thereby making itpossible to prevent the SiO₂ films 30 from being deposited on the sidesurfaces of the irregular pattern. Consequently, it is possible to omitthe step of removing the SiO₂ films 30 on the side surfaces of theirregular pattern by etching.

[0125] FIGS. 12 (a) and 12 (b) are schematic sectional views showing amethod of forming the irregular pattern in a reversed mesa shape on thesurface of the underlying GaN layer 23.

[0126] As shown in FIG. 12 (a), the striped masks 29 composed of Ni areformed on the GaN layer 23, and the sapphire substrate 21 is theninclined and is mounted on an etching apparatus at the time of dryetching. In this state, the GaN layer 23 is etched by dry etching suchas RIE.

[0127] As shown in FIG. 12 (b), the sapphire substrate 21 is inclined inthe reverse direction. In this state, the GaN layer 23 is etched by dryetching such as RIE. The irregular pattern in the reversed mesa shapecan be thus formed on the surface of the GaN layer 23.

[0128] According to the method of fabricating the GaN basedsemiconductor layer in the present embodiment, it is possible to grow,even if the sapphire substrate 21 which differs in lattice constant fromGaN, the GaN layer 24 of high quality having no dislocations on theentire surface of the sapphire substrate 21.

[0129] When a light emitting semiconductor device such as a lightemitting diode or a semiconductor laser device composed of a GaN basedsemiconductor layer is fabricated on the GaN layer 24, it is possible toimprove light output power-current characteristic and reliability.

[0130] Although in the example shown in FIGS. 9 and 10, the sapphiresubstrate 21 is used as an insulating substrate, the sapphire substrate21 can be also replaced with another insulating substrate such as an SiCsubstrate or a spinel (MgAl₂O₄) substrate.

[0131]FIG. 13 is a schematic sectional view showing an example of asemiconductor laser device fabricated on the GaN layer formed by themethod shown in FIGS. 9 and 10.

[0132] In FIG. 13, an n-GaN layer 25, an n-InGaN crack preventing layer26, an n-AlGaN cladding layer 27, an MQW active layer 28, and a p-AlGaNcladding layer 29 are formed in this order on a GaN layer 24 formed bythe method shown in FIGS. 9 and 10. An n-AlGaN current blocking layer 31is formed except in striped regions on the p-AlGaN cladding layer 29. Ap-AlGaN cladding layer 32 and a p-GaN contact layer 33 are formed inthis order on the p-AlGaN cladding layer 29 and the n-AlGaN currentblocking layer 31. A partial region from the p-GaN contact layer 33 tothe n-GaN layer 25 is removed by etching. A p electrode 34 is formed onthe p-GaN contact layer 33, and an n electrode 35 is formed on theexposed surface of the n-GaN layer 25.

[0133] In the semiconductor laser device shown in FIG. 13, a GaN basedsemiconductor layer 36 from the n-GaN layer 25 to the p-GaN contactlayer 33 is formed on the GaN layer 24 having no dislocations.Accordingly, few dislocations exist in the GaN based semiconductor layer36. Consequently, a semiconductor laser device capable of a low-currentoperation and a low-voltage operation is obtained.

[0134] Although in the present embodiment, description was made of acase where the method of forming the nitride based semiconductor layeraccording to the present invention is applied to the semiconductor laserdevice, the method of forming the nitride based semiconductor layeraccording to the present invention is also applicable to light emittingsemiconductor devices such as a light emitting diode and the othersemiconductor devices.

[0135] Although in the example shown in FIGS. 9 and 10, the GaN layer 23is etched such that it remains on the bottom surfaces of the recesses informing the striped irregular pattern on the surface of the GaN layer23, the GaN layer 23 may be etched until the sapphire substrate 21 isexposed to the bottom surfaces of the recesses, as shown in FIGS. 14 and15, in forming the irregular pattern on the surface of the GaN layer 23.

[0136] As shown in FIGS. 14 (a) and 14 (b), an AlGaN buffer layer 22 andan undoped GaN layer 23 are successively grown on a (0001) plane of asapphire substrate 21, as in FIGS. 19 (a) and 19 (b). The thickness ofthe GaN layer 23 is approximately 0.5 μm to 5 μm, for example. Also inthis case, dislocations 37 extending in the vertical direction exist inthe GaN layer 23.

[0137] An SiO₂ film is then formed on the entire surface of the GaNlayer 23, and striped masks composed of photoresist are formed on theSiO₂ film. The SiO₂ film is etched using a hydrofluoric acid, to formstriped SiO₂ films 30, as shown in FIGS. 14 (c).

[0138] Thereafter, as shown in FIG. 14 (d), the GaN layer 23 and theAlGaN buffer layer 22 are etched until the sapphire substrate 21 isexposed using the SiO₂ films 30 as masks and by RIE using chlorine gas,to form a striped irregular pattern on the surface of the GaN layer 23.The respective widths D of a recess and a projection in the irregularpattern shall be 5 μm, for example.

[0139] Thereafter, as shown in FIG. 15 (e), the GaN layer 24 is regrown.At this time, the underlying GaN layer 23 is exposed only to the sidesurfaces of the irregular pattern. When the regrowth of the GaN layer 24is started, therefore, the GaN layer 24 is not grown in the verticaldirection and is grown only in the lateral direction. No dislocationsexist in the GaN layer 24 which is grown in the lateral direction on thesapphire substrate 21.

[0140] As shown in FIG. 15 (f), as the GaN layer 24 is regrown, therecesses in the irregular pattern are filled in with the GaN layer 24.The GaN layer 24 is grown in the vertical direction.

[0141] Thereafter, as shown in FIG. 15 (g), the GaN layer 24 is grown inthe lateral direction as well as the vertical direction on the SiO₂films 30 on the top surfaces of the projections in the irregularpattern. Accordingly, the surface of the GaN layer 24 is flattened.Consequently, the GaN layer 24 of high quality having no dislocations isformed on the SiO₂ films 30 on the irregular pattern and the sapphiresubstrate 21.

[0142] As in the example shown in FIG. 9 and 10, in order to flatten thesurface of the GaN layer 24 which is regrown, the GaN layer 24 must havea certain degree of thickness. The thickness required to flatten thesurface of the GaN layer 24 differs depending on the growth conditionssuch as the width of the irregular pattern on the underlying GaN layer23 and the substrate temperature at the time of the growth of the GaNlayer 24. For example, when the respective widths of the recess and theprojection in the irregular pattern are 5 μm, the thickness of the GaNlayer 24 must be approximately 10 to 20 μm.

[0143] GaN is easily grown in a <1100> direction. In order that the GaNlayer 24 is easily grown in the lateral direction in the steps shown inFIGS. 15 (e), 15 (f), and 15 (g), therefore, it is desirable that thestriped SiO₂ films 30 are formed in a <1120> direction perpendicular tothe <1100> direction of the GaN layer 23 in the step shown in FIG. 4(c).

[0144] Furthermore, in order that the surface of the GaN layer 24 whichis regrown in the step shown in FIG. 15 (g) is easily flattened, thewidth of each of the striped SiO₂ films 30 formed in the step shown inFIG. 14 (c) and the window width of the striped SiO₂ films 30 (the widthof a region where the SiO₂ films do not exist) are preferably as smallas not more than 10 μm, and more preferably not more than 1 to 5 μm.

[0145] It is preferable that a cross-sectional shape of the projectionin the irregular pattern on the GaN layer 23 is a rectangular shape or areversed mesa shape having vertical side surfaces rather than a mesashape, as in the example shown in FIGS. 9 and 10.

[0146] Although in the example shown in FIG. 14 and 15, the sapphiresubstrate 21 is used as an insulating substrate, the sapphire substrate21 may be also replaced with another insulating substrate such as aspinel substrate.

[0147]FIGS. 16 and 17 are schematic sectional views showing the steps ofa method of fabricating a semiconductor laser device in a thirdembodiment of the present invention.

[0148] As shown in FIG. 16 (a), an AlGaN buffer layer 42 having athickness of 30 Å, an undoped GaN layer 43 having a thickness of 2 μm,and an Si doped n-GaN layer 44 a having a thickness of 3 μm are grown inthis order on a (0001) plane of a sapphire substrate 41.

[0149] As shown in FIG. 16 (b), an SiO₂ film having a thickness ofapproximately 1000 Å is then formed on the n-GaN layer 44 a. The SiO₂film in a region excluding a light emitting portion is then removed byetching, and the SiO₂ film in a region corresponding to the lightemitting portion is patterned in a stripe shape, to form a plurality ofstriped SiO₂ films 45. In this case, the striped SiO₂ films 45 areformed in a <1120> direction of the n-GaN layer 44 a such that GaN iseasily grown in the lateral direction in the subsequent step.

[0150] The width of each of the striped SiO₂ films 45 is approximately0.5 μm, and a pitch between the striped SiO₂ films 45 is approximately 1μm. In order to realize fundamental transverse mode lasing, the width W1of the light emitting portion is preferably about 2 to 5 μm, and thenumber of the striped SiO₂ films 45 must be approximately 3 to 5.

[0151] Thereafter, as shown in FIG. 16 (c), an Si doped n-GaN layer 44 bhaving a thickness of 5 μm, an Si doped n-InGaN crack preventing layer46 having a thickness of 0.1 μm, and an Si doped n-AlGaN cladding layer47 having a thickness of 1 μm are grown in this order on the n-GaN layer44 a. Further, an MQW active layer 48 having the structure shown in FIG.7, an Mg doped p-AlGaN cladding layer 49 having a thickness of 1 μm, andan Mg doped p-GaN contact layer 50 having a thickness of 0.1 μm aregrown in this order on the n-AlGaN cladding layer 47.

[0152] In this case, in a region where the striped SiO₂ films 45 exist,GaN is grown in the vertical direction from the underlying n-GaN layer44 a between the striped SiO₂ films 45, and GaN is then grown in thelateral direction on the striped SiO₂ films 45. On the other hand, in aregion where the striped SiO₂ films 45 do not exist, GaN is grown in thevertical direction from the underlying n-GaN layer 44 a. Consequently, adifference substantially arises in the growth rate of GaN between theregion where the striped SiO₂ films 45 exist and the region where thestriped SiO₂ films 45 do not exist. That is, in the region where thestriped SiO₂ films 45 exist, the growth rate of GaN is substantiallylower, as compared with that in the region where the striped SiO₂ films45 do not exist. The difference in the growth rate is continued untilthe striped SiO₂ films 45 are completely filled in with GaN so that thegrowth in the lateral direction is completed.

[0153] As a result, the surface of the n-GaN layer 44 b is concavelyformed. Further, the n-InGaN crack preventing layer 46, the n-AlGaNcladding layer 47, the MQW active layer 48, the p-AlGaN cladding layer49, and the p-GaN contact layer 50 are concavely formed. A concave partof the MQW active layer 48 is a light emitting portion which is anactive region of the device. Further, few crystal defects exist in a GaNbased semiconductor layer 56 on the region where the striped SiO₂ films45 exist.

[0154] Thereafter, as shown in FIG. 17 (d), a partial region from thep-GaN contact layer 50 to the n-GaN layer 44 b is removed by etching, toexpose the n-GaN layer 44 a.

[0155] Furthermore, as shown in FIG. 17 (e), an SiO₂ film 51 is formedon the top surface and side surfaces of the p-GaN contact layer 50 andthe top surface of the n-GaN layer 44 a in order to narrow a current andprotect an exposed portion of a pn junction except in a region above thelight emitting portion and an electrode forming region of the n-GaNlayer 44 a.

[0156] Finally, as shown in FIG. 17 (f), a p electrode 52 is formed onthe exposed surface of the p-GaN contact layer 50, and an n electrode 53is formed on the exposed surface of the n-GaN layer 44 a.

[0157] In order that a current is not uselessly injected into the lightemitting portion, it is preferable that the window width W2 of the SiO₂film 51 to be a current injection region is made equal to or slightlysmaller than the width W1 of the light emitting portion.

[0158] In a method of fabricating the semiconductor laser deviceaccording to the present embodiment, the crystallizability of a GaNbased semiconductor layer 56 on the region where the striped SiO₂ films45 exist, and the MQW active layer 48 in the light emitting portion isconcavely formed by a difference in the growth rate which arises duringthe growth in the lateral direction of GaN.

[0159] Consequently, a difference in the index of refraction appears innot only the vertical direction but also the horizontal direction of thesapphire substrate 41. As a result, it is possible to easily fabricate arefractive index guided structure by performing crystal growth twicewithout carrying out the etching step. Consequently, a refractive indexguided semiconductor laser device which is uniform in devicecharacteristics and is high in reproducibility is realized.

[0160] Although in the present embodiment, description was made of acase where the present invention is applied to the semiconductor laserdevice, the present invention is also applicable to light emittingsemiconductor devices such as a light emitting diode and the othersemiconductor devices.

[0161] Although in the third embodiment, description was made of thesemiconductor laser device using the sapphire substrate 41, it ispossible to use substrates such as an SiC substrate and a spinel(MgAl₂O₄) substrate in place of the sapphire substrate 41.

[0162] Although in the above-mentioned first, second and thirdembodiments, the SiO₂ films 4, 30, and 45 are used as insulating filmsfor carrying out epilaxial lateral overgrowth, the SiO₂ film may bereplaced with other insulating films such as an Al₂O₃ film and a TiO₂film.

[0163] Although in the above-mentioned first, second and thirdembodiments, the nitride based semiconductor layer contains Ga, Al, andIn, the nitride based semiconductor layer may contain boron (B).

[0164] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A semiconductor device comprising: a galliumarsenide substrate; a first electrode layer formed on said galliumarsenide substrate; a nitride based semiconductor layer formed on saidfirst electrode layer and containing at least one of boron, gallium,aluminum and indium; and a second electrode layer formed on said nitridebased semiconductor layer.
 2. The semiconductor device according toclaim 1, wherein said nitride based semiconductor layer includes anactive layer.
 3. The semiconductor device according to claim 2, whereinsaid nitride based semiconductor layer has a striped current injectionregion for injecting a current into said active layer, said stripedcurrent injection region is formed along a <1100> direction of saidnitride based semiconductor layer, and said nitride based semiconductorlayer is arranged on said gallium arsenide semiconductor such that the<1100> direction of said nitride based semiconductor layer coincideswith a <110> direction or a <110> direction of said gallium arsenidesubstrate, and a pair of cavity facets is formed of a {110} plane or a{110} plane of said gallium arsenide substrate and a {1100} plane ofsaid nitride based semiconductor layer.
 4. A method of fabricating asemiconductor device, comprising the steps of: forming a first nitridebased semiconductor layer containing at least one of boron, gallium,aluminum and indium on an insulating substrate; forming an insulatingfilm in a predetermined region on said first nitride based semiconductorlayer; forming a second nitride based semiconductor layer containing atleast one of boron, gallium, aluminum and indium using epitaxial lateralovergrowth on said first nitride based semiconductor layer and saidinsulating film; removing said second nitride based semiconductor layerexcept in a region on said insulating film; joining the top surface ofsaid second nitride based semiconductor layer on said insulating film toone surface of a gallium arsenide substrate through a first electrodelayer; removing said insulating film, to remove said insulatingsubstrate and said nitride based semiconductor layer from said secondnitride based semiconductor layer; and forming a second electrode layeron said second nitride based semiconductor layer.
 5. The methodaccording to claim 4, wherein the step of forming said second nitridebased semiconductor layer comprises the step of forming an active layer.6. The method according to claim 5, wherein the step of forming saidsecond nitride based semiconductor layer further comprises the step offorming a striped current injection region for injecting a current intosaid active layer along a <1100> direction of said second nitride basedsemiconductor layer, and the step of joining the top surface of saidsecond nitride based semiconductor layer to one surface of the galliumarsenide substrate through the first electrode layer comprises the stepof matching the <1100> direction of said second nitride basedsemiconductor layer with a <110> direction or a <110> direction of saidgallium arsenide substrate, said fabricating method further comprisingthe step of forming a pair of cavity facets by cleavage along a {110}plane or a {110} plane of said gallium arsenide substrate and a {1100}plane of said second nitride based semiconductor layer.
 7. A method offorming a nitride based semiconductor layer, comprising the steps of:forming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate;forming an irregular pattern including a recess having a bottom surfaceformed of an insulator and a projection having a top surface formed ofan insulator in the surface of said first nitride based semiconductorlayer; and forming a second nitride based semiconductor layer containingat least one of boron, gallium, aluminum and indium on said insulatorsby growth from said first nitride based semiconductor layer usingepitaxial lateral overgrowth.
 8. The method according to claim 7,wherein the step of forming said irregular pattern comprises the stepsof forming said irregular pattern such that said first nitride basedsemiconductor layer is exposed to the bottom surface of said recess, andforming an insulating film as said insulator on the bottom surface ofthe recess of said irregular pattern and forming an insulating film assaid insulator on the top surface of the projection of said irregularpattern.
 9. The method according to claim 7, wherein the step of formingsaid irregular pattern comprises the step of forming an insulating filmas said insulator in a region on said first nitride based semiconductorlayer where the projection of said irregular pattern is to be formed andremoving said first nitride based semiconductor such that saidinsulating substrate is exposed as said insulator except in a region onsaid insulating film.
 10. The method according to claim 7, wherein saidirregular pattern has a striped recess and a striped projection whichextend along a <1120> direction of said first nitride basedsemiconductor layer.
 11. The method according to claim 7, wherein across-sectional shape of the projection of said irregular pattern is arectangular shape or a reversed mesa shape having vertical sidesurfaces.
 12. A semiconductor device comprising: an insulatingsubstrate; a first nitride based semiconductor layer formed on saidinsulating substrate and containing at least one of boron, gallium,aluminum and indium; an irregular pattern being formed in the surface ofsaid first nitride based semiconductor layer; insulating filmsrespectively formed on the bottom surface of a recess and the topsurface of a projection of said irregular pattern of said first nitridebased semiconductor layer; and a second nitride based semiconductorlayer formed on said insulating films and containing at least one ofboron, gallium, aluminum and indium.
 13. The semiconductor deviceaccording to claim 12, wherein said second nitride based semiconductorlayer includes an active layer.
 14. A semiconductor device comprising:an insulating substrate; a first nitride based semiconductor layerformed on said insulating substrate and containing at least one ofboron, gallium, aluminum and indium; a plurality of striped insulatingfilms formed a predetermined distance away from each other on said firstnitride based semiconductor layer; and a second nitride basedsemiconductor layer formed on said first nitride based semiconductorlayer and said plurality of striped insulating films and containing atleast one of boron, gallium, aluminum and indium, said second nitridebased semiconductor layer including an active region of the device abovesaid plurality of striped insulating films.
 15. The semiconductor deviceaccording to claim 14, wherein said active region includes a lightemitting portion.
 16. The semiconductor device according to claim 15,wherein said second nitride based semiconductor layer has a stripedcurrent injection region for injecting a current into said lightemitting portion, and said second nitride based semiconductor layercomprises a pair of cavity facets perpendicular to said striped currentinjection region.
 17. The semiconductor device according to claim 15,wherein said plurality of striped insulating films are formed along a<1120> direction of said first nitride based semiconductor layer.
 18. Amethod of fabricating a semiconductor device, comprising the steps of:forming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate,forming a plurality of striped insulating films a predetermined distanceaway from each other on said first nitride based semiconductor layer;and forming a second nitride based semiconductor layer containing atleast one of boron, gallium, aluminum and indium using epitaxial lateralovergrowth on said first nitride based semiconductor layer and saidplurality of striped insulating films, to form an active region of thedevice above said plurality of striped insulating films.
 19. A method offorming a nitride based semiconductor layer, comprising the steps of:forming a first nitride based semiconductor layer containing at leastone of boron, gallium, aluminum and indium on an insulating substrate;forming an irregular pattern having exposed side surfaces in said firstnitride based semiconductor layer; and forming a second nitride basedsemiconductor layer containing at least one of boron, gallium, aluminumand indium on said irregular pattern by growth from said exposed sidesurfaces of said irregular pattern on said first nitride basedsemiconductor layer using epitaxial lateral overgrowth.
 20. The methodaccording to claim 19, wherein the step of forming said irregularpattern comprises the steps of forming said irregular pattern such thatsaid first nitride based semiconductor layer is exposed to the bottomsurface of a recess, and forming insulating films on the bottom surfaceof the recess of said irregular pattern and the top surface of theprojection of the irregular pattern.
 21. The method according to claim19, wherein the step of forming said irregular pattern comprises thestep of forming an insulating film in a region on said first nitridebased semiconductor layer where the projection of said irregular patternis to be formed, and removing said first nitride based semiconductorsuch that said insulating substrate is exposed except in a region onsaid insulating film.